Gene therapy attempts to treat diseases caused by congenital or acquired genetic defects, namely gene disorders, by substituting or supplementing defective genes with normal genes. Although various treatment methods for gene therapy have been investigated, only a very limited number of the methods to date have met with success, including the treatment of adenosine deaminase (ADA) deficiency. This is mainly because the methods for efficiently introducing a therapeutic gene into target cells and the methods for expressing an introduced gene in the cell have not yet been established. So far, liposomes, HVJ-liposomes, retroviruses, and the like have been employed as carriers introducing the therapeutic gene into target cells. However, none of them are satisfactory in gene introduction efficiency. Various attempts have been made to increase the expression efficiency of the introduced gene, by, for example, improving the promoter. However, in each case the expression efficiency of the desired gene was still poor, and the quantity of the gene product was insufficient to afford gene therapy. Thus, in the field of gene therapy, a vector that enables a high level expression of a therapeutic gene in a variety of target cells has been sought.
In the field of hematology, Tec tyrosine kinase, a protein thought to participate in the proliferation of hematopoietic stem cells, is highly expressed in mouse liver, and is also expressed in the kidney, heart, and ovary (Oncogene, 5, 1781-1786 (1990)). In humans, Tec tyrosine kinase is highly expressed in a wide range of blood and lymphoid cells (LEUKEMIA, 8, 1663-1672 (1994)).